Channel Rastering
In order to simplify the frequency search or the so-called initial cell search the center frequency of a radio channel is specified to be an integral multiple of a well-defined, generally fixed number, called channel raster. This enables a User Equipment (UE) to tune its local oscillator only at one of the raster points assuming it to be the center frequency of the channel being searched.
For example, the channel raster in UTRAN Frequency Division Duplex (FDD) is 200 kHz, but for certain channels and bands the raster is also 100 kHz. In E-UTRAN FDD and TDD, channel raster for all channels (i.e. all bandwidths) is 100 kHz. The channel raster directly impacts the channel numbering, which is described herein below.
Channel Numbering of Frequency Bands
The carrier frequencies in a frequency band are enumerated. The enumeration is standardized such that the combination of the frequency band and the carrier frequency can be determined by a unique number called absolute radio frequency number.
In GSM, UTRAN and E-UTRAN the channel numbers are called Absolute Radio Frequency Channel Number (ARFCN), UTRA Absolute Radio Frequency Channel Number (UARFCN) and E-UTRA Absolute Radio Frequency Channel Number (EARFCN), respectively.
In FDD systems, separate channel numbers are specified for UpLink (UL) and DownLink (DL). In Time Division Duplex (TDD), there is only one channel number since the same frequency is used in both directions.
The channel numbers (e.g. EARFCN) for each band are unique to distinguish between different bands. The channel number for each band can be derived from the expressions and mapping tables defined for different RATs. Based on the signaled channel numbers (e.g. EARFCN in E-UTRAN) and the pre-defined parameters associated with each band, the UE can determine the actual carrier frequency in MHz and the corresponding frequency band. This is explained by the following example.
For example, the relation between the EARFCN and the carrier frequency (FDL) in MHz for the downlink is pre-defined by the following equation:FDL=FDL_low+0.1(NDL−NOffs-DL)where FDL_low and NOffs-DL are pre-defined values for each band and NDL is the downlink EARFCN.
Consider, for example, E-UTRA band 5, whose EARFNC range (NDL) lies between 2400-2649. The pre-defined values of FDL_low and NOffs-DL are 869 and 2400, respectively. Assume the network signals downlink EARFCN to be 2500. Using the above expression, the UE can determine that the downlink carrier frequency of the channel is 879 MHz. Furthermore, as stated above that the pre-defined EARFNC range being unique for each band, hence the UE can determine the frequency band corresponding to the signaled EARFNC. An expression to derive the E-UTRA FDD uplink carrier frequency, which is similar to that of the downlink carrier frequency, is also pre-defined. In E-UTRA FDD, both fixed transmit-receive frequency separation (i.e. fixed duplex) and variable transmit-receive frequency separation (i.e. variable duplex) are supported. If fixed transmit-receive frequency separation is used by the network then the network does not have to signal the uplink EARFCN since the UE can determine the UL carrier frequency from the downlink carrier frequency and the pre-defined duplex gap. In the event that the variable duplex is employed by the network for a certain band, then both DL and UL EARFCN have to be signaled.
Channel Number Indication Mechanism for Radio Management Tasks
The network signals the channel number of a carrier along with other information to the UE for one or more radio management tasks e.g. to request UE to do measurements, for performing cell change etc. This is explained below:
For the initial cell search or more specifically for the initial carrier frequency search, the UE has to search at all possible raster frequencies e.g. with 100 kHz resolution in E-UTRAN frequency band. However, for the UEs camped on or connected to the cell, the network signals the absolute radio frequency channel number(s) for performing measurements, mobility decisions such as cell reselection or commanding handover to certain cell belonging to certain frequency channel of the same or of different RAT etc.
Hence the UE, after camping on a cell in idle mode or when connected to a cell in connected mode, can acquire the cell specific or UE specific system information, which contains information such as frequency band number (frequency band indicator), absolute radio frequency channel number(s) etc. More specifically, in LTE the band number and the ARFCN (e.g. UL EARFNC in LTE) is signaled to the UE over the relevant System Information Blocks (SIB). For example in LTE the band number and the EARFCN of the cell are signaled to the UE over SIB1 and SIB2, respectively. This information is used by the UE for performing measurements, e.g. mobility measurements, positioning measurements, Minimization of Drive Tests (MDT) measurements etc.
The network can request the UE to perform cell change (e.g. handover) to another frequency or another RAT in the frequency band, which can either be the same or different than the carrier frequency of the serving cell. Therefore, in order to assist the UE to perform the inter-frequency or inter-RAT handover, the network signals the frequency channel number of the target carrier frequency in the cell change or mobility command (e.g. handover command) to the UE.
Frequency Error in Base Station (BS) Transmitter
The carrier frequency on which the base station transmits signals on a cell may incorporate frequency error due to RF impairments.
Frequency error is the measure of the difference between the actual base station transmitted frequency and the assigned frequency. The same source is generally used for RF frequency and data clock generation.
Frequency Error in LTE BS
The modulated carrier frequency of each E-UTRA carrier configured by the BS is within the accuracy range given in Table 1 observed over a period of one subframe in LTE (1 ms). The same requirements apply to Multi-Standard Radio (MSR) base station supporting LTE.
TABLE 1Frequency error minimum requirement in LTE (FDD and TDD)BS classAccuracyWide Area BS±0.05 ppmLocal Area BS ±0.1 ppmHome BS±0.25 ppmFrequency Error in WCDMA BS
The modulated carrier frequency of the BS shall be accurate to within the accuracy range given in Table 2 observed over a period of one timeslot in WCDMA. The same requirements apply to Multi-Standard Radio (MSR) base station supporting WCDMA.
TABLE 2Frequency error minimum requirement in WCDMABS classAccuracyWide Area BS±0.05 ppmMedium Range BS ±0.1 ppmLocal Area BS ±0.1 ppmHome BS±0.25 ppm